A wind turbine blade of the airfoil type is shaped like a typical airplane wing, where the cord plane width of the blade increases continuously with decreasing distance from the hub. This might result in problems when mounting the blade to the hub and, furthermore, it causes great loads during operation of the blade due to the large surface area of the blade. Over the years, the construction of the blades has developed towards a shape, where the blades comprise a root region close to the hub, an airfoil region comprising a lift generating profile furthest away from the hub, and a transition region between the root region and the airfoil region. The transition region has a shape changing gradually from the circular shape of the root region to an airfoil profile of the airfoil region. Typically, the width of the blade in the transition region increases substantially linearly with increasing distance from the hub.
During the operating of a wind turbine, there is a risk that the blades will buckle due to the forces applied to the blades, when they rotate, and specifically in the region where the edges are bonded together. Especially in regions where the leading edge and the trailing edge are glued, there is a risk of stress concentration, and the risk of fracture is quite high. Further, the risk of fracture is increasing when the size of the blades increases. By the standard size of wind turbine blades, the problem is typically solved by placing several layers of balsa wood in the shells in order to strengthen the blade. When the number of layers of balsa wood or foam increases, the weight and costs of the blade will increase correspondingly. Further, the balsa wood or the foam will also absorb resin, which will contribute significantly to the weight of the blade and also to the production costs. For large-size blades, an extra web/beam reinforcement running from the inner side of the shells might even be incorporated, said web/beam obviously increasing the production complexity making it more expensive and also contributing to an increased weight of the blade.
EP 1310351 provides a method for making wind turbine blades in one piece without any glue joints. The problems disclosed in the prior art when using glue joints are inter alia problems with tolerances of the glue joint dimensions and difficulties with subsequent inspection of the quality of the glue joint. However, glue joints or similar bonding possibilities are necessary steps when the blades are manufactured by at least two shell parts and connected to each other in a trailing edge and a leading edge, where at least one of the edges, or at least a part of one of the edges, is applied a binder or similar adhering material on one of the shell parts.
US 2009/0068017A1 discloses a wind turbine blade comprising a spar connected to a root sub-assembly. The spar supports frame members of fibre-reinforced plastic and shaped to largely correspond to the cross-sectional profile of the blade. Skin panels of fibre-reinforced plastic are mounted on the frame members to form the outer surface of the blade. The frame members have less longitudinal extent than transverse extent and are arranged with considerable spacing.
WO 2008/003330 A1 discloses a wind turbine blade assembled by gluing an upper and a lower part of the blade together in the vicinity of the trailing edge of the blade. The upper and lower parts are attached to a U-shaped or C-shaped assembly element at the trailing edge. A deformable trailing edge section is connected to the assembly element.
EP 2119909A1 discloses a wind turbine blade formed by two longitudinally extending and mutually joined blade sections. The blade sections are joined by means of a central insert and by means of inserts at the leading and trailing edge.
DE 4225599 discloses a wind turbine blade comprising a suction side plate and a pressure side plate mutually connected at a trailing edge of the blade through a profiled member.
NL 9100816 discloses a method for assembling and reinforcing a hollow structure formed of shell halves, such as a wind turbine blade formed of two shell halves, and wherein the shell halves are assembled together with a polymer-impregnated fibre layer wrapped around an inflatable bag arranged between them, whereafter the fibre layer is pressed against the inside surfaces of the shell halves by inflating the bag. The pressure is maintained until the polymer has hardened and the shell halves thus being assembled. The bag can be filled with a foam material while being under pressure.